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The Influence of Backing Materials on Gilt Leather Microclimates
Bruce Julian
Introduction
A brief review of available literature on the subject of gilt leather reveals that there is
much done in the way of scientific research on its composition and manufacture as well as
documentation of conservation efforts performed on gilt leather samples in the past. However
literature on gilt leather tends to focus less on theory but instead specific conservation efforts
performed in the past. The connection between scientific study and practical application of
theory in the conservation of gilt leather is a desirable outcome in order to contribute to both
academic information as well as conservation methodology. The goal of this research project is
to obtain a relationship between the response time of gilt leather with respect to the variation of
backing materials behind the leather wall hangings and the effect of this relationship on
preservation purposes.
Background
Figure 1. Gilt leather on display at the Frans Hal Museum in Haarlem.
An interesting aspect of gilt leather conservation is the environment in which samples are
often kept. Gilt leather panels often remain in place within historic homes, castles, and other
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private collections. This can mean that the rooms in which the leather is kept lack a year-round,
active climate control system. Instead, interior temperature and relative humidity fluctuate in
relation to the fluctuations in the environment surrounding the building. However, heating may
be used in the winter time for human comfort purposes during meetings of conservators or
visitors to the collection. Without the addition of moisture to the air, the relative humidity of the
environment surrounding the collection will drop inversely to the rise in temperature. Studies of
the Wawel Castle in Cracow, Poland found that the relative humidity in the castle drops to
merely 45% on average.1 In addition, meetings can be periodic and of irregular length such that
the cyclical fluctuations of relative humidity pose a great risk to the gilt leather. Deformation or
fracture in the form of cracks and tears occur to release stress caused by the expansion or
contraction of the leather when it is restrained.
Leather is a hygroscopic material, meaning that it can attract and hold water molecules
from the surrounding environment until it reaches equilibrium with the air around it. This means
that the properties of hygroscopic materials change gradually as the relative humidity of the
environment changes. Increases in environmental relative humidity correspond to increases in
moisture content within the leather sample as well corresponding increases in mass and
dimensions, and vice versa. The time it takes for an object to go from one equilibrium moisture
content to another due to a step change in relative humidity in this context is called the response
time. Response time is an important characteristic which is affected by many factors such as the
object’s diffusion, permeability, and solubility qualities as well as environmental conditions.
Much of the mechanical damage experienced by leather is due to fluctuations of temperature
which result in relative humidity fluctuations in the atmosphere. These fluctuations cause the
leather to expand as humidity increases and shrink as humidity decreases. The leather is usually
stapled or nailed to their frames meaning that they are in a fully restrained state and cannot move
about freely. The aforementioned expansion and contraction cycles can cause deformation
damage over time as the physical forces in the leather can be large enough to disrupt panels from
their frames, cause paint to crack, as well as form tears in a gilt leather panel.
It has been experimentally determined that the optimum moisture content for the
conservation of leather lies in the range of 10-16%.2 This moisture level corresponds to an
atmospheric relative humidity range between 55-70%. This relative humidity (RH) range is
important because if the environmental RH drops below 55%, prolonged exposure can lead to
embrittlement of the leather due to a loss of moisture from the collagen fiber network. However
completely dry leather cannot by completely rehydrated due to hysteresis, or the dependence of
an object’s qualities on the cumulative history of storage of said object. Storage at a relative
humidity of 65% or higher can lead to the growth of mold as well as the advancement of other
1
Kozlowski, Roman, and Jaroslaw Adamowicz. "Flattening In Situ the Gilt Leather Wall Hangings." The
Conservation of Gilt Leather. Print.
2
Ibid
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biological deterioration.3 Most spores only germinate and grow at a relative humidity of 70% or
higher. Dirt or grease upon the leather provides growth media for mold. In addition the chemical
decay of cellulosic materials by hydrolysis speeds up as RH increases. For these reasons gilt
leather should be stored in the relative humidity range 55-70% to maintain optimum conditions
for conservation.
A significant consideration in the conservation of gilt leather is the control of
microclimatic conditions in the environment where the leather is kept. This is important as the
observed damage of the leather in the Wawel castle was found to be caused by uncontrolled
microclimatic conditions nearby the leather. One considerable factor in the formation of
microclimates surrounding gilt leather is the temperature of the inner walls of a historic home or
castle. The average relative humidity of the room may be maintained in the prescribed range for
leather, while the area of space directly behind the gilt leather may form microclimates much
different than the rest of the room due to the temperature and moisture gradients between the
front and back of the leather. The lack of insulation of exterior walls and their subsequent low
surface temperature can allow the walls to reach the dew point temperature of the environment.
This can cause condensation on the walls, and the high moisture levels of the microclimate that
result can prove detrimental to the condition of the gilt leather for reasons previously discussed.
This research project was developed with the previously discussed considerations in mind
in order to get a better idea of the microclimatic conditions surrounding real gilt leather. With
this study we hope to gain better insight into the variables which contribute to gilt leather
microclimates and possibly form practical applications of backing materials in order to promote
better conservation methodologies.
3
Merritt, Jane. "Mold: Prevention of Growth in Museum Collections." Conserve O Gram 3 (2007). National Park
Service. Web.
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Procedure
Figure 2. Leather samples within a climate control box.
In order to gain insight into the influence of backing materials on the microclimates
surrounding gilt leather several tests were carried out. The first of these tests involved the
establishing of hygroscopic sorption curves for one sample of gilt leather as well as various calf
leather samples. A sorption isotherm is the relationship between the equilibrium moisture content
in a material and the relative humidity of ambient air at constant temperature. This was plotted
by acclimating the samples inside of a climate control chamber to a set relative humidity starting
at 30% and a temperature of 20° Celsius until the equilibrium moisture content was achieved, as
shown in Figure 2. Next the relative humidity was increased in 10% intervals while the mass of
each sample was monitored periodically. The final change in mass of the sample was then
recorded once it had reached equilibrium and plotted versus the corresponding relative humidity.
Equilibrium was defined by a mass percentage change of less than 0.1% over a 24 hour period as
defined by EN ISO 12571 which is the standard for the determination of hygroscopic sorption
properties of building materials.
The next test performed was to measure the response times of the various leather
samples. This was achieved through first acclimating samples to a temperature of 20° C and a
relative humidity of 40% inside the climate control chamber. After the equilibrium mass was
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achieved the relative humidity of the chamber was changed immediately to a level of 80%. The
mass of the samples was monitored periodically and the percent change in mass was calculated
until the samples had once again reached equilibrium. Then the relative humidity was changed
back to 40% and the change in mass was recorded until equilibrium was achieved again. The
sorption half-times for absorption and desorption were calculated using the data collected. This
information is useful as the response times for calf leather and actual gilt leather could be
compared and analyzed for use in the final mockup.
Figure 3. Gilt leather mockup mounted to cool wall within large climatic control chamber.
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Figure 4. Humbugs mounted within mockup frame.
The last portion of the experiment involved the building of a gilt leather mockup in order
to better understand the microclimates that form around it. This was achieved through the use of
a 40x40 cm square frame constructed of pine upon which calf leather was tensioned and secured
with staples. The frame was then mounted to the cooling wall within the large climatic control
chamber, as seen in Figure 2. The temperature of the cool wall could be varied in order to
simulate the inside surface of an exterior wall. The temperature and relative humidity of the
room was then varied as well to simulate the cyclical heating and cooling that a real panel of gilt
leather might encounter in a historic home. The temperature and humidity directly in front of and
behind the leather was monitored periodically with the use of “humbugs”, or Hanwell sensors
which record the temperature and relative humidity of the environment as seen in Figure 4. The
humbugs were set to record measurements every five minutes. Once the temperature and
humidity on both sides of the leather matched the leather was said to be in equilibrium. Finally
once testing had been completed various backing materials were placed behind the mockup in
order to observe and measure the influence each material would have. The cavity closest to the
leather was named Cavity 1 and the cavity closest to the wall was named Cavity 2.
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Figure 9. Sensor accuracy and percent differences
Initial testing of the climate control chamber revealed that there were discrepancies
between the various sensors placed within the room. This could be a result of the differences in
sensor placement around the room as well as the fact the air is not completely mixed. In order to
get an idea of the calibration of each sensor the climate chamber was set to a relative humidity
50% and a temperature of 20° C, then readings were taken from each sensor at the same time to
calculate the differences. Since the Hanwell 4000 was the newest and most recently calibrated
sensor it was used as the reference value and the percent difference with the other sensors were
calculated. After referencing relevant literature for each sensor it can be seen that the humbugs
are within the error bounds of the manufacturer, and thus all readings taken during tests should
be representative of the actual conditions in the climate control room.
Results
Gilt Leather
26.0%
Mass change (%)
24.0%
22.0%
20.0%
18.0%
16.0%
14.0%
12.0%
10.0%
20
30
40
50
60
70
80
RH (%)
Figure 5. Gilt leather sorption isotherm from 30-80% relative humidity.
90
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Calf Leather
26.0%
Mass change (%)
24.0%
22.0%
20.0%
18.0%
16.0%
14.0%
12.0%
10.0%
0
10
20
30
40
50
60
70
80
Relative Humidity (%)
Figure 6. Calf leather sorption isotherm from 40-80% relative humidity.
Upon establishment of the sorption isotherms for both the gilt leather and the calf leather
samples it is evident that the gilt leather can absorb moisture from the environment to a much
higher degree than the calf leather for the same amount of material. Both samples began the test
with the same mass up to the fourth significant figure when subject to a relative humidity of
30%. Therefore any changes in mass after changing the relative humidity can be attributed to
increased water activity within the leather samples. This data suggests that the gilt leather is has
greater permeability and solubility properties with respect to humidity when compared to new
calf leather. This is important because the results of subsequent tests using calf leather will be
smaller in magnitude than the same test using gilt leather. The shape of each graph is in line with
reference information for leather isotherms in that they are nearly linear.
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Absorption
10.0%
Mass change (%)
8.0%
6.0%
Control 1
4.0%
Gilt Leather
2.0%
Control 2
0.0%
-2.0%
0.0
2.0
4.0
6.0
8.0
10.0
Time (Days)
Figure 7. Absorption response time testing for gilt leather and control samples
Desorption
10.0%
Mass change (%)
8.0%
6.0%
Control 1
4.0%
Gilt Leather
Control 2
2.0%
0.0%
0.0
-2.0%
2.0
4.0
6.0
8.0
10.0
Time (Days)
Figure 8. Desorption response time testing for gilt leather and control samples
From the response time testing of the gilt and calf leather it is evident that the aged gilt
leather has a much faster response time than the calf leather. We can see from the data that the
gilt leather reaches equilibrium about three times faster than the calf leather for absorption and
about 1.3 times faster for desorption. The hygroscopic properties of materials vary widely from
one another and even in the same material a wide variation is frequently observed, and is
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dependent upon the prior history of the material. This difference in the response times of the
leather samples is important as further testing performed with calf leather can be correlated to the
behavior of actual gilt leather under the same conditions.
RH
Figures 9, 10, 11. Cavity temperature and humidity data obtained from mock up frame testing.
From the initial testing of the mockup frame within the climate control chamber it is
evident that the cavity closest to the wall, Cavity 2, is much more stable in terms of temperature
and humidity than Cavity 1 when backing materials are placed within the frame. Cavity 1
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responds much more quickly to changes in the room’s temperature and relative humidity due to
its separation from the cool wall by the backing material. In this way we can see that the cool
wall acts to stabilize the microclimate directly behind the leather by increasing the time it takes
to equilibrate to the climate of the room around the leather. We can also see that the wood
backing material actually increases the relative humidity of Cavity 2 during heating. In both
backing materials tests we can see that the relative humidity of Cavity 1 near the leather drops to
a much lower level during heating than when the backing materials are not present, to about 35%
with backing materials and about 45% without them.
Conclusions and Future Recommendations
From the data collected in both preliminary tests and the leather mockup it can be seen
that the use of backing materials decreases the amount of time it takes for the microclimate
directly behind the leather to respond to changes in temperature and humidity of the room. This
can be a desirable characteristic if the temperature and relative humidity of the room are kept
within the recommended range for gilt leather constantly. However if the room is heated and the
relative humidity is not maintained through the addition of moisture, the use of backing materials
actually decreases the relative humidity behind the leather to even lower levels than without
them. This can be detrimental to the leather in that it would make the cycling of relative humidity
around the leather even greater in magnitude, potentially leading to even greater mechanical
damage due to the leather being constrained. This problem would be even more exaggerated in
the case of real gilt leather with its much faster response time in comparison to the calf leather
used in the test. For this reason it can be concluded that the use of backing materials is
recommended for gilt leather kept within rooms with full time relative humidity control. In the
absence of such humidity regulation then the use of backing materials would not be
recommended.
Future recommendations for this project would include the use of surface temperature
sensors on the cool wall, leather, and backing materials. This would be useful in seeing whether
or not dew point temperatures are reached within the mockup and give a greater understanding of
the microclimatic conditions within the cavities. Cold climates and minimal insulation in the
walls of many building can cause interior surfaces to reach the dew point leading to condensation
and high humidity levels.4 Further testing of the mockup might also benefit from the use of
larger changes in temperature of the climate control room as well as the cool wall. This would
likely increase the changes in microclimatic conditions within the mockup to make differences in
results due to the variation of backing materials more evident. Further testing could also be
performed on the dimensional changes of the leather due to relative humidity changes in order to
4
Mecklenburg, Marion. "Micro Climates and Moisture Induced Damage to Paintings." Smithsonian Museum
Conservation Institute. Web.
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understand the cyclical stresses the leather experiences, as well as performing tensile testing to
understand the elasticity properties of the gilt leather.